The present invention relates to fiber reinforced thermoplastics which are modified to reduce shrinkage and warpage. More particularly, the invention relates to fiber reinforced thermoplastic composites modified with silicone interpenetrating polymer networks.
For many years, it has been known that the mechanical properties of thermoplastics could be significantly increased by the addition of reinforcing fibers. Traditionally, such fibers have included glass, carbon and mineral fibers, while more recently more sophisticated fibers such as aramid, ceramic, boron and metal fibers have been used for special applications.
A serious detrimental effect of reinforcing fibers in thermoplastic resins is the tendency of reinforced thermoplastic composites to warp after molding (post-warpage). Moreover, the introduction of reinforcing fibers in the thermoplastic resin aggravates the problem of differential mold shrinkage, namely the tendency of the resin to shrink more in the direction transverse to the direction of flow than in the direction of flow. This differential is apparently due to the tendency of the reinforcing fibers to align themselves generally in the direction of flow of the resin melt and is at least partially responsible for the post-warpage.
Increased differential shrinkage and warpage are particularly serious problems in the production of high tolerance parts, such as gears. These are also serious problems in the molding of thin walled and variable thickness parts. Further, in thick cross section moldings (i.e. 1/4 inch or greater thickness), the differential shrinkage causes high stress formation, resulting in stress cracking and voids. In these and other applications, it is highly desirable to have a minimum of shrinkage and warpage, as well as essentially isotropic shrinkage behavior.
In the past several years, Petrarch Systems Inc. of Bristol, Pennsylvania has developed a new class of melt processable thermoplastics which contain silicone interpenetrating polymer networks (IPN's). These compositions are sometimes referred to as pseudo-interpenetrating polymer networks (pseudo- or semi-IPN's) since only the silicone component is vulcanized or cross-linked in the thermoplastic matrix. The interpenetrating polymer network is formed by vulcanizing a hydride functional silicone group and a polymer containing at least one unsaturated group, preferably a vinyl group, during normal thermoplastic melt processing. See U.S. Pat. No. 4,500,688 and pending U.S. Pat. application Ser. No. 577,113 of Barry C. Arkles.
The pseudo- or semi-IPN's have the advantages of exhibiting surface and di-electric properties which approach those of silicones and mechanical properties which approach those of the thermoplastic matrix. In many cases, properties such as wear, lubricity and heat and abrasion resistance of the thermoplastics are improved. While this technology was originally developed in polyurethane systems, it has been extended to other thermoplastic elastomers and engineering thermoplastics. For example, pending U.S. Pat. application Ser. No. 577,113 of Petrarch Systems Inc. discloses that polyamide (nylon) and polyester engineering thermoplastics have been formulated into materials which appear to be particularly well suited for gear and bearing applications, with the heat distortion temperature of nylon 6,6 being increased by the incorporation of 5 wt % silicone IPN.
However, the major uses of silicone IPN's have still been in the modification of lower modulus thermoplastics, particularly elastomers. To applicant's knowledge, the silicone IPN's have not been used or recognized to improve the shrinkage and/or warpage characteristics of high modulus thermoplastic materials, i.e., thermoplastic materials which in the unfilled state have a flexural modulus greater than 90,000 psi, as measured by ASTM D790.